A barrel is also used to measure liquids. One barrel of oil is about 158.987 litres (42 US gallons). It is shortened as "b". This use may have come from the early Pennsylvanian oil fields where wooden barrels were used to store and move oil around. In 1866, the people of West Virginia started using it regularly. At this time, the amount was set to 42 gallons. It was later world-wide for measuring amounts of oil and pricing it. Today, oil is rarely stored or transported in actual barrels, but the term "barrel" is still used to mean a 42-gallon measurement.

A barrel can also be used to measure dry goods. For example, a barrel can mean 196 pounds (89 kilograms) of flour.

The first people to start using barrels were the Celts about 350 BC. The Roman Empire started using barrels after trading with the Celts. Barrels remained popular for shipping items until the 20th century.